I'm just about to step outside and go march, but one last update: it's huge. My boss just called me...he's at the end of Avenida Libertador, about 7 km. out from the starting point. Says there's asolid crowd there. The TV images show that there's a solid crowd stretching all the way back to the Parque del Este starting point: the people at the back haven't been able to even start walking, and those . That's 7 solid kilometers of wide streets packed solid with marchers (that's 4.4 miles, for the metrically impaired.)
As of 12:15 pm, the march remains fully peaceful. Fingers crossed.